1. Field of the Invention:
This invention relates to a camera having an interchangeable lens.
2. Description of the Related Art:
Recently, various types of video apparatus including video cameras are increasingly diffused. For example, in the field of video cameras, interchangeable lens systems have been proposed which are designed to use interchangeable lenses for the purpose of increasing functions and which are also designed for personal or home use.
Video cameras have many functions including an automatic focusing function.
The principle of the operation of an automatic focusing system will be described below by way of example.
A type of automatic focusing system for video cameras is known in which the definition of an imaged video picture is detected by the magnitude of high-frequency components of a video signal, and the position of the lens is controlled so that the definition (high frequency components) is maximized, thereby automatically controlling focusing of the camera. More specifically, the image signal abruptly changes at an edge of the image of the object so that high-frequency components of the signal increase. If the high-frequency components increase, it is judged that the image becomes closer to the focused state. In relation to this system, a method of determining the direction in which a focus motor is moved if the lens is deviated from the focusing position to a non-focusing position is known in which a piezoelectric element is attached to a lens or an imaging part and is vibrated finely to effect focus modulation, and judgment is made to determine whether the phase of this vibration and the phase of a change in the high-frequency components of the video signal coincide with each other (near-focus) or opposite to each other (far-focus), thereby determining the focus lens driving direction. (This method is called a modulation method.)
In a case where an interchangeable lens system is applied to such a video camera, it is necessary to consider functions provided in a lens unit and processes relating to these functions as well as functions provided in a camera unit and processes relating to these functions, in particular, timing control in order to achieve overall functions of the camera equivalent to those of the conventional camera including the automatic focusing function.
However, in a case where an interchangeable lens system is designed based on the existing video camera systems, there is a need for a process of making the lens unit and the camera unit operate while maintaining a particular phase/timing relationship therebetween. For example, it is necessary for the automatic focusing means to detect a change in a signal representing the degree of focusing in phase with small vibration of the focusing lens of the lens unit. Where lenses which are simply detachable are adopted, there is a possibility of failure to perform such a process with accuracy.
To solve this problem, a method may be adopted which provides, in the lens unit or the camera unit, an automatic focusing system in a complete form whose functions are distributed to only one of the lens unit and the camera unit, e.g., an active system in which reflection of a projection light such as infrared is received by a light receiving element and focusing is performed by utilizing triangulation or a TTL system in which incident light introduced through the lens is received by a special sensor and focusing is performed in the same manner. However, if such a system is adopted, the design for interchangeable lenses is considerably limited and substantial part of the constructions and the control procedures of the lens unit and the camera unit cannot be fully utilized, resulting in failure to utilize the advantages of lens changing.
Also, signal lines may be provided for the number of categories of data necessary for realization of the functions of the lens unit as well as the functions of the camera unit. However, the number of common signal connections is thereby increased. It is difficult to achieve the desired functions if the connection signal system has already been determined and if the increase in the number of signals is limited.